Replacement gate electrode with planar work function material layers

ABSTRACT

In a replacement gate scheme, a continuous material layer is deposited on a bottom surface and a sidewall surface in a gate cavity. A vertical portion of the continuous material layer is removed to form a gate component of which a vertical portion does not extend to a top of the gate cavity. The gate component can be employed as a gate dielectric or a work function material portion to form a gate structure that enhances performance of a replacement gate field effect transistor.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.13/186,597, filed Jul. 20, 2011 the entire content and disclosure ofwhich is incorporated herein by reference.

BACKGROUND

The present disclosure relates to semiconductor structures, andparticularly to a metal-oxide-semiconductor field effect transistor(MOSFET) having a high performance replacement gate electrode configuredto provide reduced parasitic capacitance and/or low resistance, andmethods of manufacturing the same.

A replacement gate metal-oxide-semiconductor field effect transistor(MOSFET) can accommodate a high dielectric constant (high-k) gatedielectric material that is prone to degradation at high temperature dueto decomposition or other structural degradation mechanisms. Areplacement gate MOSFET is formed by forming activated source and drainregions and optionally metal semiconductor alloys before deposition of agate dielectric and a gate electrode. A replacement gate MOSFET employsa recessed region, which is typically referred to as a “gate cavity,”that is subsequently filled with a gate dielectric and a gate electrode.The recessed region is typically formed by removing a disposable gatestructure. Because the gate dielectric and the gate electrode “replaces”the disposable gate structure by filling the gate cavity, the gatedielectric material, which is typically a high-k gate dielectricmaterial, follows the contour of the recessed region.

A challenge in employing the replacement gate scheme to manufacture highperformance devices is to fill gate cavities with a conductive materialhaving a high conductivity. The overall conductivity of replacement gateconductor structures is limited due to the relatively high conductivityof materials employed as work function material layers. While theoptimal work function material layers can provide appropriate workfunction levels for p-type field effect transistors and n-type fieldeffect transistors, respectively, such work function material layers donot provide as high conductivity as a conductive fill material thatfills the remaining portion of a gate cavity. Further, the presence ofthe work function material layers on sidewalls of gate cavities reducesthe width of the gate cavities so that the volume that the conductivefill material can occupy is reduced. In addition, reduction of widthwith scaling in conjunction with the presence of work function materiallayers on sidewalls of gate cavities can cause formation of voids duringthe filling of the gate cavities with the conductive fill material. Thecombination of the above factors contributes to a significant increasein the resistivity of gate conductors in replacement gate structures asdevice scaling continues, and limits performance of advanced replacementgate field effect transistors.

BRIEF SUMMARY

Replacement gate work function material stacks are provided, whichprovide a work function about the energy level of the conduction band ofsilicon. After removal of a disposable gate stack, a gate dielectriclayer is formed in a gate cavity. A metallic layer including a metal anda non-metal element is deposited directly on the gate dielectric layer.At least one barrier layer and a metal layer are deposited andplanarized to fill the gate cavity. The metallic layer includes amaterial having a work function about 4.0 eV, and specifically, lessthan 4.4 eV, and can include a material selected from tantalum carbideand a hafnium-silicon alloy. Thus, the metallic layer can provide a workfunction that enhances the performance of an n-type field effecttransistor employing a silicon channel.

According to an aspect of the present disclosure, a method of forming asemiconductor structure is provided. The method includes: forming a gatecavity laterally surrounded by a planarization dielectric layer on asemiconductor substrate, wherein a top surface of the semiconductorsubstrate is exposed at a bottom of the gate cavity; forming a gatedielectric layer in the gate cavity; forming at least one planar workfunction material portion having a topmost surface that is recessed froma topmost surface of the planarization dielectric layer on the gatedielectric layer in the gate cavity; and filling the gate cavity with ametal layer contacting the at least one planar work function materialportion.

According to another aspect of the present disclosure, a semiconductorstructure is provided, which includes: a planarization dielectric layerhaving a planar topmost surface and located on a semiconductorsubstrate; a U-shaped gate dielectric located on the semiconductorsubstrate and embedded in the planarization dielectric layer; at leastone planar work function material portion located within the U-shapedgate dielectric and having a topmost surface that is recessed from atopmost surface of the planarization dielectric layer; and a metalportion having a top surface that is coplanar with the top surface ofthe planarization dielectric layer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is vertical cross-sectional view of an exemplary semiconductorstructure after formation of disposable gate level layers according toan embodiment of the present disclosure.

FIG. 2 is a vertical cross-sectional view of the exemplary semiconductorstructure after patterning of disposable gate structures and formationof source/drain extension regions according to an embodiment of thepresent disclosure.

FIG. 3 is a vertical cross-sectional view of the exemplary semiconductorstructure after formation of source/drain trenches according to anembodiment of the present disclosure.

FIG. 4 is a vertical cross-sectional view of the exemplary semiconductorstructure after formation of embedded stress-generating source/drainregions according to an embodiment of the present disclosure.

FIG. 5 is a vertical cross-sectional view of the exemplary semiconductorstructure after deposition and planarization of a planarizationdielectric layer according to an embodiment of the present disclosure.

FIG. 6 is a vertical cross-sectional view of the exemplary semiconductorstructure after removal of the disposable gate structures according toan embodiment of the present disclosure.

FIG. 7 is a vertical cross-sectional view of the exemplary semiconductorstructure after formation of a gate dielectric layer according to anembodiment of the present disclosure.

FIG. 8 is a vertical cross-sectional view of the exemplary semiconductorstructure after formation of a first work function material layeraccording to an embodiment of the present disclosure.

FIG. 9 is a vertical cross-sectional view of the exemplary semiconductorstructure after a first etch back process according to an embodiment ofthe present disclosure.

FIG. 10 is a vertical cross-sectional view of the exemplarysemiconductor structure after removal of remaining portions of the firstwork function material layer from a second type field effect transistorregion according to an embodiment of the present disclosure.

FIG. 11 is a vertical cross-sectional view of the exemplarysemiconductor structure after formation of a second work functionmaterial layer according to an embodiment of the present disclosure.

FIG. 12 is a vertical cross-sectional view of the exemplarysemiconductor structure after a second etch back process according to anembodiment of the present disclosure.

FIG. 13 is a vertical cross-sectional view of the exemplarysemiconductor structure after deposition of a conductive fill layeraccording to an embodiment of the present disclosure.

FIG. 14 is a vertical cross-sectional view of the exemplarysemiconductor structure after planarization.

FIG. 15 is a vertical cross-sectional view of the exemplarysemiconductor structure after formation of a contact level dielectriclayer and contact via structures.

DETAILED DESCRIPTION

As stated above, the present disclosure relates to semiconductorstructures having dual work function material gates and a high-k gatedielectric, and methods of manufacturing the same, which are nowdescribed in detail with accompanying figures. Like and correspondingelements mentioned herein and illustrated in the drawings are referredto by like reference numerals. The drawings are not necessarily drawn toscale.

Referring to FIG. 1, an exemplary semiconductor structure according toan embodiment of the present disclosure includes a semiconductorsubstrate 8, on which various components of field effect transistors aresubsequently formed. The semiconductor substrate 8 can be a bulksubstrate including a bulk semiconductor material throughout, or asemiconductor-on-insulator (SOI) substrate (not shown) containing a topsemiconductor layer, a buried insulator layer located under the topsemiconductor layer, and a bottom semiconductor layer located under theburied insulator layer.

Various portions of the semiconductor material in the semiconductorsubstrate 8 can be doped with electrical dopants of n-type or p-type atdifferent dopant concentration levels. For example, the semiconductorsubstrate 8 may include an underlying semiconductor layer 10, a secondconductivity type well 12A formed in a first device region (the regionon the left side in FIG. 1), and a first conductivity type well 12Bformed in a second device region (the region on the right side in FIG.1). The first conductivity type well 12B is doped with dopants of afirst conductivity type, which can be n-type or p-type. The secondconductivity type well 12A is doped with dopants of a secondconductivity type, which is the opposite of the first conductivity type.If the first conductivity type is p-type, the second conductivity typeis n-type, and vice versa.

Shallow trench isolation structures 20 are formed to laterally separateeach of the first conductivity type well 12B and the second conductivitytype well 12A. Typically, each of the first conductivity type well 12Band the second conductivity type well 12A is laterally surrounded by acontiguous portion of the shallow trench isolation structures 20. If thesemiconductor substrate 8 is a semiconductor-on-insulator substrate,bottom surfaces of the first conductivity type well 12B and the secondconductivity type well 12A may contact a buried insulator layer (notshown), which electrically isolates each of the first conductivity typewell 12B and the second conductivity type well 12A from othersemiconductor portions of the semiconductor substrate 8 in conjunctionwith the shallow trench isolation structures 20. In one embodiment,topmost surfaces of the shallow trench isolation structures can besubstantially coplanar with topmost surfaces of the first conductivitytype well 12B and the second conductivity type well 12A.

Disposable gate level layers are deposited on the semiconductorsubstrate 8 as blanket layers, i.e., as unpatterned contiguous layers.The disposable gate level layers can include, for example, a verticalstack a disposable gate dielectric layer 23L, a disposable gate materiallayer 27L, and a disposable gate cap dielectric layer 29L. Thedisposable gate dielectric layer 23L can be, for example, a layer ofsilicon oxide, silicon nitride, or silicon oxynitride. The thickness ofthe disposable gate dielectric layer 23L can be from 1 nm to 10 nm,although lesser and greater thicknesses can also be employed. Thedisposable gate material layer 27L includes a material that can besubsequently removed selective to the dielectric material of aplanarization dielectric layer to be subsequently formed. For example,the disposable gate material layer 27L can include a semiconductormaterial such as a polycrystalline semiconductor material or anamorphous semiconductor material. The thickness of the disposable gatematerial layer 27L can be from 30 nm to 300 nm, although lesser andgreater thicknesses can also be employed. The disposable gate capdielectric layer 29L can include a dielectric material such as siliconoxide, silicon nitride, or silicon oxynitride. The thickness of thedisposable gate cap dielectric layer 29L can be from 3 nm to 30 nm,although lesser and greater thicknesses can also be employed. While thepresent disclosure is illustrated with disposable gate level layersincluding a vertical stack a disposable gate dielectric layer 23L, adisposable gate material layer 27L, and a disposable gate cap dielectriclayer 29L, any other disposable gate level layers can also be employedprovided that the material(s) in the disposable gate level layers can beremoved selective to a planarization dielectric layer to be subsequentlyformed.

Referring to FIG. 2, the disposable gate level layers (29L, 27L, 23L)are lithographically patterned to form disposable gate structures.Specifically, a photoresist (not shown) is applied over the topmostsurface of the disposable gate level layers (29L, 27L, 23L) and islithographically patterned by lithographic exposure and development. Thepattern in the photoresist is transferred into the disposable gate levellayers (29L, 27L, 23L) by an etch, which can be an anisotropic etch suchas a reactive ion etch. The remaining portions of the disposable gatelevel layers (29L, 27L, 23L) after the pattern transfer constitutedisposable gate structures.

The disposable gate stacks may include, for example, a first disposablegate structure formed over the second conductivity type well 12A in thefirst device region and a second disposable gate structure formed overthe first conductivity type well 12B in the second device region. Thefirst disposable gate structure is a stack of a first disposable gatedielectric portion 23A, a first disposable gate material portion 27A,and a first disposable gate cap portion 29A, and the second disposablegate structure is a stack of a second disposable gate dielectric portion23B, a second disposable gate material portion 27B, and a seconddisposable gate cap portion 29B. The first disposable gate cap portion29A and the second disposable gate cap portion 29B are remainingportions of the disposable gate cap dielectric layer 29L. The firstdisposable gate material portion 27A and the second disposable gatematerial portion 27B are remaining portions of the disposable gatematerial layer 27L. The first disposable gate dielectric portion 23A andthe second disposable gate dielectric portion 23B are remaining portionsof the disposable gate dielectric layer 23L.

Masked ion implantations can be employed to form various source/drainextension regions. For example, dopants of the first conductivity typecan be implanted into portions of the second conductivity type well 12Athat are not covered by the first disposable gate structure (23A, 27A,29A) to form first source/drain extension regions 14A having a doping ofthe first conductivity type. The first conductivity type well 12B can bemasked by a patterned photoresist (not shown) during this implantationprocess to prevent implantation of additional dopants of the firstconductivity type therein. As used herein, “source/drain extensionregions” collectively refer to source extension regions and drainextension regions. Similarly, dopants of the second conductivity typecan be implanted into portions of the first conductivity type well 12Bthat are not covered by the second disposable gate structure (23B, 27B,29B) to form second source/drain extension regions 14B. The secondconductivity type well 12A can be masked by another patternedphotoresist (not shown) during this implantation process to preventimplantation of dopants of the second conductivity type therein.

Referring to FIG. 3, gate spacers are formed on sidewalls of each of thedisposable gate structures, for example, by deposition of a conformaldielectric material layer and an anisotropic etch. The gate spacers caninclude a first gate spacer 52A formed around the first disposable gatestructure (23A, 27A, 29A) and a second gate spacer 52B formed around thesecond disposable gate structure (23B, 27B, 29B).

In one embodiment, semiconductor material can be removed from theportions of the semiconductor substrate that are not covered by thedisposable gate structures, gate spacers (52A, 52B), or the shallowtrench isolation structures 20 to form cavities within the semiconductorsubstrate 8. For example, first source/drain cavities 13A can be formedin the first device region, and second source/drain cavities 13B can beformed in the second device region by an anisotropic etch that removesthe semiconductor materials of the second conductivity type well 12A andthe first conductivity type well 12B. The depth of the first and secondsource/drain cavities (13A, 13B) does not exceed the depths of thesecond conductivity type well 12A and the first conductivity type well12B, and preferably, does not exceed the depths of the shallow trenchisolation structures 20. The first and second source/drain cavities(13A, 13B) can have substantially vertical sidewalls that are verticallycoincident with a bottom portion of the outer sidewalls of the gatespacers (52A, 52B).

Referring to FIG. 4, a first dielectric liner (not shown) can bedeposited and lithographically patterned to cover the second deviceregion, while exposing the semiconductor surfaces on the firstsource/drain cavities 13A. First embedded stress-generating source/drainregions 16A are formed by selective epitaxy of a semiconductor materialthat is lattice mismatched relative to the semiconductor material of thesecond conductivity type well 12A.

For example, if the second conductivity type well 12A includes a p-dopedsingle crystalline silicon and an n-type field effect transistor is tobe formed in the second device region, the first embeddedstress-generating source/drain regions 16A can include an n-doped singlecrystalline silicon-carbon alloy material in which the carbonconcentration is between 0% and 2.5% in atomic concentration, which isthe solubility limit of carbon in silicon. In this case, the firstembedded stress-generating source/drain regions 16A apply a longitudinaltensile stress in channel of the n-type field effect transistor alongthe direction connecting the two first embedded stress-generatingsource/drain regions 16A. The longitudinal tensile stress along thedirection of the channel of the n-type field effect transistor increasesthe mobility of minority carriers (electrons) in the channel region, andtherefore, increases the on-current of the n-type field effecttransistor.

Alternately, if the second conductivity type well 12A includes ann-doped single crystalline silicon and a p-type field effect transistoris to be formed in the first device region, the first embeddedstress-generating source/drain regions 16A can include a p-doped singlecrystalline silicon-germanium alloy material, in which the germaniumconcentration can be between 0% and 30% in atomic concentration. In thiscase, the first embedded stress-generating source/drain regions 16Aapply a longitudinal compressive stress in channel of the p-type fieldeffect transistor along the direction connecting the two first embeddedstress-generating source/drain regions 16A. The longitudinal compressivestress along the direction of the channel of the p-type field effecttransistor increases the mobility of minority carriers (holes) in thechannel region, and therefore, increases the on-current of the p-typefield effect transistor.

A second dielectric liner (not shown) can be deposited andlithographically patterned to cover the first device region, whileexposing the semiconductor surfaces on the first source/drain cavities13A. Any remaining portion of the first dielectric liner is removed fromthe sidewalls and bottom surfaces of the second source/drain cavities13B so that semiconductor surfaces of the first conductivity type well12B are exposed at the sidewalls and bottom surfaces of the secondsource/drain cavities 13B. Second embedded stress-generatingsource/drain regions 16B are formed by selective epitaxy of asemiconductor material that is lattice mismatched relative to thesemiconductor material of the first conductivity type well 12A.

The material of the second embedded stress-generating source/drainregions 16B can provide the opposite type of stress to the type ofstress that the first embedded stress-generating source/drain regions16A generate. Thus, if the first embedded stress-generating source/drainregions 16A generate a longitudinal tensile stress, the second embeddedstress-generating source/drain regions 16B generate a longitudinalcompressive stress. For example, the first conductivity type well 12Bcan include an n-doped single crystalline silicon, and the secondembedded stress-generating source/drain regions 16B can include ap-doped single crystalline silicon-germanium alloy material, in whichthe germanium concentration can be between 0% and 30% in atomicconcentration.

Alternately, if the first embedded stress-generating source/drainregions 16A generate a longitudinal compressive stress, the secondembedded stress-generating source/drain regions 16B generate alongitudinal tensile stress. For example, the first conductivity typewell 12B can include a p-doped single crystalline silicon, and thesecond embedded stress-generating source/drain regions 16B can includean n-doped single crystalline silicon-carbon alloy material, in whichthe carbon concentration can be between 0% and 2.5% in atomicconcentration.

The topmost surfaces of the first and second embedded stress-generatingsource/drain regions 16A can be raised above, coplanar with, or recessedbelow, the plane of the bottom surfaces of the first and seconddisposable gate dielectric portions (23A, 23B) depending on the amountof the epitaxial material selectively deposited in the first and secondsource/drain cavities (13A, 13B).

Each of the first embedded stress-generating source region (one of16A's) and the first embedded stress-generating drain region (the otherof 16A's) is epitaxially aligned to a single crystalline semiconductormaterial of the second conductivity type well 12A, which subsequentlyfunctions as a body of a first field effect transistor. Each of thesecond embedded stress-generating source region (one of 16B's) and thesecond embedded stress-generating drain region (the other of 16B's) isepitaxially aligned to a single crystalline semiconductor material ofthe first conductivity type well 12B, which subsequently functions as abody of a second field effect transistor.

While the present disclosure is illustrated with an embodiment in whichembedded stress-generating source/drain regions (16A, 16B) are employed,embodiments in which one or more of the embedded stress-generatingsource/drain regions (16A, 16B) are replaced with (a) source/drainregion(s) that are formed by ion implantation of dopants can also bepracticed. In such embodiment, dopants of the first conductivity typeare implanted into portions of the second conductivity type well 12Athat are not covered by the first disposable gate structure (23A, 27A,29A) and the first gate spacer 52A to form first source and drainregions having a doping of the first conductivity type. The firstconductivity type well 12B can be masked by a photoresist (not shown)during the implantation of the first conductivity type dopants toprevent implantation of the first conductivity type dopants therein.Similarly, dopants of the second conductivity type are implanted intoportions of the first conductivity type well 12B that are not covered bythe second disposable gate structure (23B, 27B, 29B) and the second gatespacer 52B to form second source and drain regions having a doping ofthe second conductivity type. The second conductivity type well 12A canbe masked by a photoresist (not shown) during the implantation of thesecond conductivity type dopants to prevent implantation of the secondconductivity type dopants therein.

Referring to FIG. 5, first metal semiconductor alloy portions 46A andsecond metal semiconductor alloy portions 46B can be formed on exposedsemiconductor material on the top surface of the semiconductor substrate8, for example, by deposition of a metal layer (not shown) and ananneal. Unreacted portions of the metal layer are removed selective toreacted portions of the metal layer. The reacted portions of the metallayer constitute the metal semiconductor alloy portions (46A, 46B),which can include a metal silicide portions if the semiconductormaterial of the first and second embedded stress-generating source anddrain regions (16A, 16B) include silicon.

The various metal semiconductor alloy portions (46A, 46B) include afirst source-side metal semiconductor alloy portion (one of 46A's)formed on the first embedded stress-generating source region (one of16A's), a first drain-side metal semiconductor alloy portion (the otherof 16A's) formed on the first embedded stress-generating drain region(the other of 16A's), a second source-side metal semiconductor alloyportion (one of 46B's) formed on the second embedded stress-generatingsource region (one of 16B's), and a second drain-side metalsemiconductor alloy portion (the other of 16B's) formed on the secondembedded stress-generating drain region (the other of 16B's).

Optionally, a dielectric liner (not shown) may be deposited over themetal semiconductor alloy portions (46A, 46B), the first and seconddisposable gate structures (23A, 27A, 29A, 23B, 27B, 29B), and the firstand second gate spacers (52A, 52B). Optionally, a firststress-generating liner (not shown) and a second stress-generating liner(not shown) can be formed over the first disposable gate structure (23A,27A, 29A) and the second disposable gate structure (23B, 27B, 29B),respectively. The first stress-generating liner and the secondstress-generating liner can include a dielectric material that generatesa compressive stress or a tensile stress to underlying structures, andcan be silicon nitride layers deposited by plasma enhanced chemicalvapor deposition under various plasma conditions.

A planarization dielectric layer 60 can be deposited over the firststress-generating liner and/or the second stress-generating liner, ifpresent, or over the metal semiconductor alloy portions (46A, 46B), thefirst and second disposable gate structures (23A, 27A, 29A, 23B, 27B,29B), and the first and second gate spacers (52A, 52B) if (a)stress-generating liner(s) is/are not present. Preferably, theplanarization dielectric layer 60 is a dielectric material that may beeasily planarized. For example, the planarization dielectric layer 60can be a doped silicate glass or an undoped silicate glass (siliconoxide).

The planarization dielectric layer 60 and any additional dielectricmaterial layers (which include any of the first stress-generating liner,the second stress-generating liner, and the dielectric liner that arepresent, are planarized above the topmost surfaces of the first andsecond disposable gate structures (23A, 27A, 29A, 23B, 27B, 29B), i.e.,above the topmost surfaces of the first and second disposable gate capportions (29A, 29B). The planarization can be performed, for example, bychemical mechanical planarization (CMP). The planar topmost surface ofthe planarization dielectric layer 60 is herein referred to as a planardielectric surface 63. The topmost surfaces of the disposable gate capportions (29A, 29B) are coplanar with the planar dielectric surface 63after the planarization.

The combination of the first source and drain extension regions 14A, thefirst embedded stress-generating source and drain regions 16A, and thesecond conductivity type well 12A can be employed to subsequently form afirst field effect transistor. The combination of the second source anddrain extension regions 14B, the second embedded stress-generatingsource and drain regions 16B, and the first conductivity type well 12Bcan be employed to subsequently form a second type field effecttransistor.

Referring to FIG. 6, the first disposable gate structure (23A, 27A, 29A)and the second disposable gate structure (23B, 27B, 29B) are removed byat least one etch. The first and second disposable gate structures (23A,27A, 29A, 23B, 27B, 29B) can be removed, for example, by at least oneetch, which can include an anisotropic etch, an isotropic etch, or acombination thereof. The at least one etch can include a dry etch and/ora wet etch. The at least one etch employed to remove the first andsecond disposable gate structures (23A, 27A, 29A, 23B, 27B, 29B) ispreferably selective to the dielectric materials of the planarizationdielectric layer 60 and any other dielectric material layer that ispresent above the semiconductor substrate 8.

A first gate cavity 25A is formed in the volume from which the firstdisposable gate structure (23A, 27A, 29A) is removed, and a second gatecavity 25B is formed in the volume from which the second disposable gatestructure (23B, 27B, 29B) is removed. A semiconductor surface of thesemiconductor substrate 8, i.e., the top surface of the secondconductivity type well 12A, is exposed at the bottom of the first gatecavity 25A. Another semiconductor surface of the semiconductor substrate8, i.e., the top surface of the first conductivity type well 12B, isexposed at the bottom of the second gate cavity 25B. Each of the firstand second gate cavities (25A, 25B) is laterally surrounded by theplanarization dielectric layer 60. The first gate spacer 52A laterallysurrounds the first gate cavity 25A, and the second gate spacer 52Blaterally surrounds the second gate cavity 25B. The inner sidewalls ofthe first gate spacer 52A can be substantially vertical, and extendsfrom the top surface of the second conductivity type well 12A to theplanar dielectric surface 63, i.e., the topmost surface, of theplanarization dielectric layer 60. Further, the inner sidewalls of thesecond gate spacer 52B can be substantially vertical, and extends fromthe top surface of the first conductivity type well 12B to the planardielectric surface 63 of the planarization dielectric layer 60.

Referring to FIG. 7, exposed portions of the semiconductor surfaces ofthe semiconductor substrate 8 can be converted to a dielectric materiallayer. For example, a first semiconductor-element-containing dielectriclayer 31A can be formed on the exposed surface of the secondconductivity type well 12A by conversion of the exposed semiconductormaterial into a dielectric material, and a secondsemiconductor-element-containing dielectric layer 31B can be formed onthe exposed surface of the first conductivity type well 12B byconversion of the exposed semiconductor material into the dielectricmaterial. The formation of the semiconductor-element-containingdielectric layers (31A, 31B) can be effected by thermal conversion orplasma treatment. If the semiconductor material of the secondconductivity type well 12A and the first conductivity type well 12Bincludes silicon, the semiconductor-element-containing dielectric layers(31A, 31B) can include silicon oxide or silicon nitride. Thesemiconductor-element-containing dielectric layers (31A, 31B) areinterfacial dielectric layers that contact a semiconductor surfaceunderneath and gate dielectrics to be subsequently deposited thereupon.The thickness of the semiconductor-element-containing dielectric layers(31A, 31B) can be from 0.3 nm to 1.2 nm, although lesser and greaterthicknesses can also be employed.

A gate dielectric layer 32L is deposited on the bottom surfaces andsidewalls of the gate cavities (25A, 25B) and the topmost surface of theplanarization dielectric layer 60. The gate dielectric layer 32L can bea high dielectric constant (high-k) material layer having a dielectricconstant greater than 8.0. The gate dielectric layer 32L can include adielectric metal oxide, which is a high-k material containing a metaland oxygen, and is known in the art as high-k gate dielectric materials.Dielectric metal oxides can be deposited by methods well known in theart including, for example, chemical vapor deposition (CVD), physicalvapor deposition (PVD), molecular beam deposition (MBD), pulsed laserdeposition (PLD), liquid source misted chemical deposition (LSMCD),atomic layer deposition (ALD), etc. Exemplary high-k dielectric materialinclude HfO₂, ZrO₂, La₂O₃, Al₂O₃, TiO₂, SrTiO₃, LaAiO₃, Y₂O₃,HfO_(x)N_(y), ZrO_(x)N_(y), La₂O_(x)N_(y), Al₂O_(x)N_(y), TiO_(x)N_(y),SrTiO_(x)N_(y), LaAlO_(x)N_(y), Y₂O_(x)N_(y), a silicate thereof, and analloy thereof. Each value of x is independently from 0.5 to 3 and eachvalue of y is independently from 0 to 2. The thickness of the gatedielectric layer 32L, as measured at horizontal portions, can be from0.9 nm to 6 nm, and preferably from 1.0 nm to 3 nm. The gate dielectriclayer 32L may have an effective oxide thickness on the order of or lessthan 1 nm. In one embodiment, the gate dielectric layer 32L is a hafniumoxide (HfO₂) layer.

Referring to FIG. 8, a first work function material layer 34L isdeposited employing an anisotropic deposition method. The first workfunction material layer 34L includes a first metallic material having afirst work function.

For example, if the second conductivity type well 12A includes a p-dopedsingle crystalline semiconductor material and the second device regionincludes an n-type field effect transistor, the first metallic materialcan have a work function that is closer to the conduction band energylevel of the semiconductor material than to the valence band energylevel of the semiconductor material. If the second conductivity typewell 12A includes p-doped single crystalline silicon and the seconddevice region includes an n-type field effect transistor, the firstmetallic material can have a work function between the conduction bandenergy level of silicon and the mid-band gap energy level, i.e., theenergy level at the middle between the valence band edge and theconduction band edge of silicon. In this case, the first metallicmaterial can include Hf, Ti, Zr, Cd, La, Tl, Yb, Al, Ce, Eu, Li, Pb, Tb,Bi, In, Lu, Nb, Sm, V, Zr, Ga, Mg, Gd, Y, TiAl, TaN, a stack thereof, aconductive oxide thereof, a conductive nitride thereof, an alloythereof, and a combination thereof.

If the second conductivity type well 12A includes an n-doped singlecrystalline semiconductor material and the second device region includesa p-type field effect transistor, the first metallic material can have awork function that is closer to the valence band energy level of thesemiconductor material than to the conduction band energy level of thesemiconductor material. If the second conductivity type well 12Aincludes n-doped single crystalline silicon and the second device regionincludes a p-type field effect transistor, the first metallic materialcan have a work function between the valence band energy level ofsilicon and the mid-band gap energy level of silicon. In this case, thefirst metallic material can include Pt, Rh, Ir, Ru, Cu, Os, Be, Co, Pd,Te, Cr, Ni, TiN, a stack thereof, a conductive oxide thereof, aconductive nitride thereof, an alloy thereof, and a combination thereof.

The anisotropic deposition method employed to deposit the first workfunction material layer 34L can be any method that provides adirectional deposition so that more first metallic material is depositedon horizontal surfaces than on vertical surfaces. For example, theanisotropic deposition method can be a collimated physical vapordeposition (PVD) method, in which the first metallic material isdirected downward in directions substantially parallel to the verticaldirection of the exemplary semiconductor structure. Alternately, theanisotropic deposition method can employ radio frequency physical vapordeposition (RFPVD) sputtering and/or with constant voltage substratebias, i.e., constant electrical voltage bias applied to the substrate.

In one embodiment, the angular spread of the directions of the particlesof the first metallic material that are sputtered toward the exemplarysemiconductor structure is less than 30 degrees from the verticaldirection, i.e., the direction perpendicular to the topmost surface ofthe planarization dielectric layer 30, and preferably less than 15degrees from the vertical direction, and more preferably less than 7.5degrees from the vertical direction.

The anisotropy inherent in the deposition method causes the verticalportions of the first work function material layer 34L to have a lesserthickness than horizontal portions of the first work function materiallayer 34L. The ratio of the thickness of the horizontal portions of thefirst work function material layer 34L at the bottom of the first andsecond gate cavities (25A, 25B) to the thickness of the verticalportions of the first work function material layer 34L on the sidewallsof the first and second gate cavities (25A, 25B) can be greater than3.0:1, and preferably greater than 4.0:1. Thus, the horizontal portionsof the first work function material layer 34L have a thickness that isat least three times the thickness of the first vertical portions of thework function material layer 34L. Experimental collimated PVD methodsperformed for the purpose of the present disclosure have demonstratedratios ranging from 4:1 to 5:1.

The thickness of the horizontal portions of the first work functionmaterial layer 34L at the bottom of the first and second gate cavities(25A, 25B) can be from 3 nm to 15 nm, although lesser and greaterthicknesses can also be employed.

Referring to FIG. 9, a first etch back process is performed to removevertical portions of the first work function material layer 34L. Thefirst etch back process can be an isotropic etch. For example, the firstetch back process can be a wet etch that etches all exposed surfaceportions of the first work function material layer 34L at a same etchrate. Alternately, the first etch back process can be a gas phase etch.The vertical portions of the first work function material layer 34L andupper portions of horizontal portions of the first work functionmaterial layer 34L are removed by the isotropic etch.

The duration of the first etch back process is set such that allvertical portions of the first work function material layer 34L areremoved, while horizontal portions of the first work function materiallayer 34L are not completely removed. A remaining horizontal portion ofthe first work function material layer 34L within the first cavity 25Ais herein referred to as a first planar work function material portion34, and a remaining horizontal portion of the first work functionmaterial layer within the second cavity 25B is herein referred to as adisposable work function material portion 34B, which is removed in asubsequent processing step. A remaining horizontal portion of the firstwork function material layer 34L above the planarization dielectriclayer 60 is herein referred to as a first planar work function metallayer 34L′, which has a uniform thickness throughout.

The first planar work function material portion 34 has a topmost surfacethat is recessed from the topmost surface of the planarizationdielectric layer 60. The first planar work function material portion 34is formed in the first gate cavity 25A and directly on a top surface andon a bottom portion of inner sidewall surfaces of the gate dielectriclayer 32L. The first planar work function material portion 34 can have auniform thickness between one inner vertical sidewall of the gatedielectric layer 32L and another inner vertical sidewall of the gatedielectric layer 32L within the first gate cavity 25A. Inner sidewallsof the gate dielectric layer 32L are exposed above the first planar workfunction material portion 34 and the disposable work function materialportion 34B within the first gate cavity 25A and the second gate cavity25B, respectively.

The thickness of the first planar work function material portion 34 atthe bottom of the first gate cavity 25A and the disposable work functionmaterial portion 34B are the same, and can be from 2.5 nm to 10 nm,although lesser and greater thicknesses can also be employed.

Referring to FIG. 10, a photoresist 39 is applied and lithographicpatterned so that the photoresist 39 covers the area over the secondconductivity type well 12A, while the disposable work function materialportion 34B and a portion of the first planar work function metal layer34L′ are exposed over the first conductivity type well 12B. Thedisposable work function material portion 34B and the exposed portion ofthe first planar work function metal layer 34L′ are removed by an etch,which can be a wet etch or a dry etch. The photoresist 39 is removed,for example, by ashing or wet etching.

Referring to FIG. 11, a second work function material layer 36L isdeposited employing an anisotropic deposition method. The second workfunction material layer 36L includes a second metallic material having asecond work function.

For example, if the first conductivity type well 12B includes an n-dopedsingle crystalline semiconductor material and the first device regionincludes a p-type field effect transistor, the second metallic materialcan have a work function that is closer to the valence band energy levelof the semiconductor material than to the conduction band energy levelof the semiconductor material. If the first conductivity type well 12Bincludes n-doped single crystalline silicon and the first device regionincludes a p-type field effect transistor, the second metallic materialcan have a work function between the valence band energy level ofsilicon and the mid-band gap energy level of silicon. In this case, thesecond metallic material can include Pt, Rh, Ir, Ru, Cu, Os, Be, Co, Pd,Te, Cr, Ni, TiN, a stack thereof, a conductive oxide thereof, aconductive nitride thereof, an alloy thereof, and a combination thereof.

If the first conductivity type well 12B includes a p-doped singlecrystalline semiconductor material and the second device region includesan n-type field effect transistor, the second metallic material can havea work function that is closer to the conduction band energy level ofthe semiconductor material than to the valence band energy level of thesemiconductor material. If the first conductivity type well 12B includesp-doped single crystalline silicon and the second device region includesan n-type field effect transistor, the second metallic material can havea work function between the conduction band energy level of silicon andthe mid-band gap energy level of silicon. In this case, the secondmetallic material can include Hf, Ti, Zr, Cd, La, Tl, Yb, Al, Ce, Eu,Li, Pb, Tb, Bi, In, Lu, Nb, Sm, V, Zr, Ga, Mg, Gd, Y, TiAl, TaN, a stackthereof, a conductive oxide thereof, a conductive nitride thereof, analloy thereof, and a combination thereof.

The anisotropic deposition method employed to deposit the second workfunction material layer 36L can be any method that provides adirectional deposition so that more second metallic material isdeposited on horizontal surfaces than on vertical surfaces. For example,the anisotropic deposition method can be a collimated physical vapordeposition (PVD) method, in which the second metallic material isdirected downward in directions substantially parallel to the verticaldirection of the exemplary semiconductor structure.

In one embodiment, the angular spread of the directions of the particlesof the second metallic material that are sputtered toward the exemplarysemiconductor structure is less than 30 degrees from the verticaldirection, i.e., the direction perpendicular to the topmost surface ofthe planarization dielectric layer 30, and preferably less than 15degrees from the vertical direction, and more preferably less than 7.5degrees from the vertical direction.

The anisotropy inherent in the deposition method causes the verticalportions of the second work function material layer 36L to have a lesserthickness than horizontal portions of the second work function materiallayer 36L. The ratio of the thickness of the horizontal portions of thesecond work function material layer 36L at the bottom of the first andsecond gate cavities (25A, 25B) to the thickness of the verticalportions of the second work function material layer 36L on the sidewallsof the first and second gate cavities (25A, 25B) can be greater than3.0:1, and preferably greater than 4.0:1. Thus, the horizontal portionsof the second work function material layer 36L have a thickness that isat least three times the thickness of the vertical portions of thesecond work function material layer 36L.

The thickness of the horizontal portions of the second work functionmaterial layer 36L at the bottom of the first and second gate cavities(25A, 25B) can be from 3 nm to 15 nm, although lesser and greaterthicknesses can also be employed.

Referring to FIG. 12, a second etch back process is performed to removevertical portions of the second work function material layer 36L. Thesecond etch back process can be an isotropic etch. For example, thesecond etch back process can be a wet etch that etches all exposedsurface portions of the second work function material layer 36L at asame etch rate. The vertical portions of the second work functionmaterial layer 36L and upper portions of horizontal portions of thesecond work function material layer 36L are removed by the isotropicetch.

The duration of the second etch back process is set such that allvertical portions of the second work function material layer 36L areremoved, while horizontal portions of the second work function materiallayer 36L are not completely removed. A remaining horizontal portion ofthe second work function material layer 36L within the first cavity 25Ais herein referred to as a second planar work function material portion36A, and a remaining horizontal portion of the second work functionmaterial layer within the second cavity 25B is herein referred to as aplanar work function material portion 36B. A first remaining horizontalportion of the second work function material layer 36L contacts the gatedielectric layer 32L in the second device region, and a second remaininghorizontal portion of the second work function material layer 36Lcontacts the top surface of the first planar work function metal layer34L′. The first remaining horizontal portion of the second work functionmaterial layer 36L is herein referred to as a lower second planar workfunction metal layer 36L′, and the second remaining horizontal portionof the second work function material layer 36L is herein referred to asan upper second planar work function metal layer 36L″. The lower secondplanar work function metal layer 36L′ and the upper second planar workfunction metal layer 36L″ have a uniform thickness throughout. The lowersecond planar work function metal layer 36L′ may, or may not, becontiguous with the upper second planar work function metal layer 36L″depending on duration of the second etch back process.

The second planar work function material portion 36A has a topmostsurface that is recessed from the topmost surface of the planarizationdielectric layer 60. The second planar work function material portion36A is formed in the first gate cavity 25A and directly on the topsurface of the first planar work function material portion 34. Further,the second planar work function material portion 36A contacts a portionof inner sidewall surfaces of the gate dielectric layer 32L within thefirst gate cavity 25A. The second planar work function material portion36A can have a uniform thickness between one inner vertical sidewall ofthe gate dielectric layer 32L and another inner vertical sidewall of thegate dielectric layer 32L within the first gate cavity 25A. Innersidewalls of the gate dielectric layer 32L are exposed above the secondplanar work function material portion 36A within the first gate cavity25A.

The planar work function material portion 36B has a topmost surface thatis recessed from the topmost surface of the planarization dielectriclayer 60. The planar work function material portion 36B is formed in thesecond gate cavity 25B and directly on the top surface of the gatedielectric layer 32L within the second gate cavity 25B. Further, theplanar work function material portion 36B contacts a bottom portion ofinner sidewall surfaces of the gate dielectric layer 32L within thesecond gate cavity 25B. The second planar work function material portion36B can have a uniform thickness between one inner vertical sidewall ofthe gate dielectric layer 32L and another inner vertical sidewall of thegate dielectric layer 32L within the second gate cavity 25B. Innersidewalls of the gate dielectric layer 32L are exposed above the planarwork function material portion 36B within the second gate cavity 25B.

Thus, at least one planar work function material portion, i.e., theplanar work function material portion 36B or the combination of thefirst planar work function material portion 34 and the second planarwork function material portion 36A, having a topmost surface that isrecessed from the topmost surface of the planarization dielectric layer60 is formed on the gate dielectric layer 32L in each of the first andsecond gate cavities (25A, 25B). Each of the at least one planar workfunction can have a uniform thickness between one inner verticalsidewall of the gate dielectric layer 32L and another inner verticalsidewall of the gate dielectric layer 32L.

The thickness of the second planar work function material portion 36A atthe bottom of the first gate cavity 25A and the planar work functionmaterial portion 36A at the bottom of the second gate cavity 25B can bethe same, and can be from 2.5 nm to 10 nm, although lesser and greaterthicknesses can also be employed.

Referring to FIG. 13, an optional barrier metal layer 38L can bedeposited on the exposed surfaces of the lower second planar workfunction metal layer 36L′, the upper second planar work function metallayer 36L″, the second planar work function material portion 34A, theplanar work function material portion 34B, and the sidewalls of the gatedielectric layer 32L within the first and second gate cavities (25A,25B). In a non-limiting illustrative example, the optional barrier metallayer 38L can include a tantalum nitride layer, a titanium nitridelayer, a titanium-aluminum alloy, or a combination thereof. Thethickness of the optional barrier metal layer 38L can be from 0.5 nm to20 nm, although lesser and greater thicknesses can also be employed. Theoptional barrier metal layer 38L may be omitted in some embodiments. Inone embodiment, the optional barrier metal layer 38L includes a metallicnitride. For example, the optional barrier metal layer 38L can includetitanium nitride.

The gate cavities (25A, 25B) are filled with a metal layer 40L. In anembodiment in which a barrier metal layer 40L is employed, the metallayer 40L can be deposited directly on the optional barrier metal layer38L. In an embodiment in which a barrier metal layer is not employed,the metal layer 40L can be deposited directly on the exposed surfaces ofthe lower second planar work function metal layer 36L′, the upper secondplanar work function metal layer 36L″, the second planar work functionmaterial portion 34A, the planar work function material portion 34B, andthe sidewalls of the gate dielectric layer 32L within the first andsecond gate cavities (25A, 25B).

The metal layer 40L can include a metal deposited by physical vapordeposition or chemical vapor deposition. For example, the metal layer40L can be an aluminum layer or an aluminum alloy layer deposited byphysical vapor deposition. The thickness of the metal layer 40L, asmeasured in a planar region of the metal layer 40L above the top surfaceof the planarization dielectric layer 60, can be from 100 nm to 500 nm,although lesser and greater thicknesses can also be employed. In oneembodiment, the metal layer 40L consists essentially of a singleelemental metal such as Al, Au, Ag, Cu, or W. For example, the metallayer 40L can consist essentially of aluminum.

Referring to FIG. 14, the metal layer 40L, the optional barrier metallayer 38L, the lower second planar work function metal layer 36L′, theupper second planar work function metal layer 36L″, and the gatedielectric layer 32L are planarized, for example, by chemical mechanicalplanarization. Specifically, portions of the metal layer 40L, theoptional barrier metal layer 38L, the lower second planar work functionmetal layer 36L′, the upper second planar work function metal layer36L″, and the gate dielectric layer 32L are removed from above theplanar dielectric surface 63 of the planarization dielectric layer 60 atthe end of the planarization step. The remaining portion of the gatedielectric layer 32L in the first device region forms a first gatedielectric 32A, and the remaining portion of the gate dielectric layer32L in the second device region forms a second gate dielectric 32B. Theremaining portion of the optional barrier metal layer 38L in the firstdevice region forms a first optional barrier metal portion 38A, and theremaining portion of the optional barrier layer in the second deviceregion forms a second optional barrier metal portion 38B. The remainingportion of the metal layer 40L in the first device region constitutes afirst metal portion 40A, and the remaining portion of the metal layer40L in the second deice region constitutes a second metal portion 40B.The topmost surfaces of the first and second gate dielectrics (32A,32B), the first and second optional barrier metal portions (38A, 38B),and the first and second metal portions (40A, 40B) are coplanar with thetopmost surface of the planarization dielectric layer 60.

Thus, replacement gate stacks are formed within the volume previouslyoccupied by the first and second gate cavities (25A, 25B) at the step ofFIG. 6. The replacement gate stacks include a first replacement gatestack 230A located in the first device region and a second replacementgate stack 230B located in the second device region. Each replacementgate stack (230A, 230B) overlies a channel region of a field effecttransistor. The first replacement gate stack 230A and the secondreplacement gate stack 230B are formed concurrently.

A first field effect transistor is formed in the first device region.The first field effect transistor includes the second conductivity typewell 12A, the first source/drain extension regions 14A, the firstembedded stress-generating source/drain regions 16A, the first metalsemiconductor alloy portions 46A, the first replacement gate stack 230A,and the first gate spacer 52A. The first replacement gate stack 230Aincludes the optional first semiconductor-element-containing dielectriclayer 31A, the first gate dielectric 32A, the first planar work functionmaterial portion 34, the second planar work function material portion36A, the first optional barrier metal portion 38A, and the first metalportion 40A.

A second field effect transistor is formed in the second device region.The second field effect transistor includes the first conductivity typewell 12B, the second source/drain extension regions 14B, the secondembedded stress-generating source/drain regions 16B, second metalsemiconductor alloy portions 46B, the second replacement gate stack230B, and the second gate spacer 52B. The second replacement gate stack230B includes the optional second semiconductor-element-containingdielectric layer 31B, the second gate dielectric 32B, the work functionmaterial portion 36B, the second optional barrier metal portion 38B, andthe second metal portion 40B. The second planar work function materialportion 36A in the first replacement gate stack 230A and the workfunction material portion 36B in the second replacement gate stack 230Bhave the same material composition and the same thickness.

Each of the first and second gate dielectrics (32A, 32B) is a U-shapedgate dielectric, which includes a horizontal gate dielectric portion anda vertical gate dielectric portion extending upward from peripheralregions of the horizontal gate dielectric portion. In the first fieldeffect transistor, the first work function material portion 34 and thesecond work function material portion contact inner sidewalls of thevertical gate dielectric portion of the first gate dielectric 32A. Inthe second field effect transistor, the work function material portion36B contacts inner sidewalls of the vertical gate dielectric portion ofthe second gate dielectric 32B. Each U-shaped gate dielectric is locatedon the semiconductor substrate 8 and is embedded in the planarizationdielectric layer 60. At least one planar work function material portionis located within each U-shaped gate dielectric. The at least one planarwork function material portion can be a planar work function materialportion 36B located within the second gate dielectric 32B, or can be avertical stack of the first planar work function material portion 34 andthe second planar work function material portion 36A.

Each of the at least one planar work function material portion and has atopmost surface that is recessed from the topmost surface of theplanarization dielectric layer 60. The topmost surface of the at leastone planar work function material portion can be a horizontal surfacethat extends from one inner sidewall of a U-shaped gate dielectric toanother inner sidewall of the U-shaped gate dielectric. Each of the atleast one planar work function material portion, i.e., each of the firstplanar work function material portion 34, the second planar workfunction material portion 36A, and the planar work function materialportion 36B, can have a uniform thickness between one inner verticalsidewall of a U-shaped gate dielectric and another inner verticalsidewall of the U-shaped gate dielectric.

Each gate dielectric (32A, 32B), as a U-shaped gate dielectric, includesa horizontal gate dielectric portion and a vertical gate dielectricportion. The vertical gate dielectric portion contiguously extends fromthe horizontal gate dielectric portion to the topmost surface of theplanarization dielectric layer 60. If the first and second barrier metalportions (38A, 38B) are present, the entirety of inner sidewalls of eachU-shaped gate dielectric (32A, 32B) contacts sidewalls of the at leastone planar work function material portion and first and second barriermetal portions (38A, 38B). If first and second barrier metal portionsare not present, the entirety of inner sidewalls of each U-shaped gatedielectric (32A, 32B) contacts sidewalls of the at least one planar workfunction material portion and a metal portion, which is either the firstmetal portion 40A or the second metal portion 40B. Inner sidewalls of agate spacer (52A, 52B) contacts outer sidewalls of each gate dielectric.

Each of the at least one planar work function material portion (34, 36A,36B) can include a metallic material selected from Pt, Rh, Ir, Ru, Cu,Os, Be, Co, Pd, Te, Cr, Ni, TiN, Hf, Ti, Zr, Cd, La, Tl, Yb, Al, Ce, Eu,Li, Pb, Tb, Bi, In, Lu, Nb, Sm, V, Zr, Ga, Mg, Gd, Y, TiAl, TaN, a stackthereof, a conductive oxide thereof, a conductive nitride thereof, analloy thereof, and a combination thereof. Within the stack of the firstplanar work function material portion 34 and the second planar workfunction material portion 36A, the first planar work function materialportion 34 includes a first metallic material and the second planar workfunction material portion 36A includes a second material. In oneembodiment, the first material can be selected from Pt, Rh, Ir, Ru, Cu,Os, Be, Co, Pd, Te, Cr, Ni, TiN, a stack thereof, a conductive oxidethereof, a conductive nitride thereof, an alloy thereof, and acombination thereof, and the second metallic material can be selectedfrom Hf, Ti, Zr, Cd, La, Tl, Yb, Al, Ce, Eu, Li, Pb, Tb, Bi, In, Lu, Nb,Sm, V, Zr, Ga, Mg, Gd, Y, TiAl, TaN, a stack thereof, a conductive oxidethereof, a conductive nitride thereof, an alloy thereof, and acombination thereof. In another embodiment, the first material can beselected from Hf, Ti, Zr, Cd, La, Tl, Yb, Al, Ce, Eu, Li, Pb, Tb, Bi,In, Lu, Nb, Sm, V, Zr, Ga, Mg, Gd, Y, TiAl, TaN, a stack thereof, aconductive oxide thereof, a conductive nitride thereof, an alloythereof, and a combination thereof, and the second metallic material canbe selected from Pt, Rh, Ir, Ru, Cu, Os, Be, Co, Pd, Te, Cr, Ni, TiN, astack thereof, a conductive oxide thereof, a conductive nitride thereof,an alloy thereof, and a combination thereof.

Referring to FIG. 15, a contact level dielectric layer 70 is depositedover the planarization dielectric layer 60. Various contact viastructures can be formed, for example, by formation of contact viacavities by a combination of lithographic patterning and an anisotropicetch followed by deposition of a conductive material and planarizationthat removes an excess portion of the conductive material from above thecontact level dielectric layer 70. The various contact via structurescan include, for example, first source/drain contact via structures(i.e., at least one first source contact via structure and at least onefirst drain contact via structure) 66A, second source/drain contact viastructures (i.e., at least one second source contact via structure andat least one second drain contact via structure) 66B, a first gatecontact via structure 68A, and a second gate contact via structure 68B.Each source contact via structure (66A, 66B) and each drain contact viastructure (66A, 66B) are embedded in the planarization dielectric layer60 and the contact level dielectric material layer 70. Each sourcecontact via structure (one of 66A and 66B) contacts a source-side metalsemiconductor alloy portion (one of 46A and 46B), and each drain contactvia structure (another of 66A and 66B) contacts a drain-side metalsemiconductor alloy portion (another of 46A and 46B).

While the disclosure has been described in terms of specificembodiments, it is evident in view of the foregoing description thatnumerous alternatives, modifications and variations will be apparent tothose skilled in the art. Accordingly, the disclosure is intended toencompass all such alternatives, modifications and variations which fallwithin the scope and spirit of the disclosure and the following claims.

What is claimed is:
 1. A semiconductor structure comprising: aplanarization dielectric layer having a planar topmost surface andlocated on a semiconductor substrate; a first gate stack comprising: aU-shaped gate dielectric located on a bottom and sidewalls of a firstgate cavity laterally surrounded by said planarization dielectric layerand located in a first device region of said semiconductor substrate; afirst planar work function material portion comprising a first metallicmaterial having a first work function and present on a horizontalportion of said U-shaped gate dielectric; a second planar work functionmaterial portion present on said first planar work function materialportion, said second planar work function material portion comprising asecond metallic material having a second work function different fromsaid first metallic material and having a topmost surface that isrecessed from said planar topmost surface of said planarizationdielectric layer; and a metal portion present on said topmost surface ofsaid second planar work function material portion, said metal portionhaving a top surface that is coplanar with said planar topmost surfaceof said planarization dielectric layer; and a second gate stackcomprising: another U-shaped gate dielectric located on a bottom andsidewalls of a second gate cavity laterally surrounded by saidplanarization dielectric layer and located in a second device region ofsaid semiconductor substrate; another second planar work functionmaterial portion present on a horizontal portion of said anotherU-shaped gate dielectric, said another second planar work functionmaterial portion having a topmost surface that is recessed from saidplanar topmost surface of said planarization dielectric layer, andanother metal portion present on said topmost surface of said anothersecond planar work function material portion, said another metal portionhaving a top surface that is coplanar with said planar topmost surfaceof said planarization dielectric layer.
 2. The semiconductor structureof claim 1, wherein each of said first and said second planar workfunction material portions has a uniform thickness between one innervertical sidewall of said U-shaped gate dielectric and another innervertical sidewall of said U-shaped gate dielectric, and wherein saidanother second planar work function material portion has a uniformthickness between one inner vertical sidewall of said another U-shapedgate dielectric and another inner vertical sidewall of said anotherU-shaped gate dielectric.
 3. The semiconductor structure of claim 1,wherein said first metallic material is selected from Pt, Rh, Ir, Ru,Cu, Os, Be, Co, Pd, Te, Cr, Ni, TiN, a stack thereof, a conductive oxidethereof, a conductive nitride thereof, an alloy thereof, and acombination thereof, and wherein said second metallic material isselected from Hf, Ti, Zr, Cd, La, Tl, Yb, Al, Ce, Eu, Li, Pb, Tb, Bi,In, Lu, Nb, Sm, V, Zr, Ga, Mg, Gd, Y, TiAl, TaN, a stack thereof, aconductive oxide thereof, a conductive nitride thereof, an alloythereof, and a combination thereof.
 4. The semiconductor structure ofclaim 1, wherein each of said U-shaped gate dielectric and said anotherU-shaped gate dielectric includes the horizontal portion and a verticalportion that contiguously extends from said horizontal portion to saidplanar topmost surface of said planarization dielectric layer.
 5. Thesemiconductor structure of claim 1, wherein said topmost surface of saidsecond planar work function material portion is a horizontal surfacethat extends from one inner sidewall of said U-shaped gate dielectric toanother inner sidewall of said U-shaped gate dielectric, and whereinsaid topmost surface of said another second planar work functionmaterial portion is a horizontal surface that extends from one innersidewall of said another U-shaped gate dielectric to another innersidewall of said another U-shaped gate dielectric.
 6. The semiconductorstructure of claim 1, wherein an entirety of inner sidewalls of saidU-shaped gate dielectric contacts sidewalls of said first and saidsecond planar work function material portions and said metal portion,and wherein an entirety of inner sidewalls of said another U-shaped gatedielectric contacts sidewalls of said another second planar workfunction material portion and said another metal portion.
 7. Thesemiconductor structure of claim 1 further comprising a gate spacerhaving inner sidewalls that contact outer sidewalls of said U-shapedgate dielectric, and another gate spacer having inner sidewalls thatcontact outer sidewalls of said another U-shaped gate dielectric.
 8. Thesemiconductor structure of claim 1, wherein sidewalls of said firstplanar work function material portion and sidewalls of said secondplanar work function material portion contact a vertical portion of saidU-shaped gate dielectric, and wherein sidewalls of said another secondplanar work function material portion contact a vertical portion of saidanother U-shaped gate dielectric.
 9. The semiconductor structure ofclaim 1, further comprising: an embedded stress-generating source regionand an embedded stress-generating drain region that are epitaxiallyaligned to a single crystalline semiconductor material of a body of afield effect transistor in said first device region or a body of anotherfield effect transistor in said second device region.
 10. Thesemiconductor structure of claim 9, further comprising; a source-sidemetal semiconductor alloy portion and a drain-side metal semiconductoralloy portion located on said embedded stress-generating source regionand said embedded stress-generating drain region, respectively.
 11. Thesemiconductor structure of claim 10, further comprising: a contact leveldielectric material layer located above said planarization dielectriclayer.
 12. The semiconductor structure of claim 11, further comprising asource contact via structure and a drain contact via structure embeddedin said planarization dielectric layer and said contact level dielectricmaterial layer and contacting said source-side metal semiconductor alloyportion and said drain-side metal semiconductor alloy portion,respectively.
 13. The semiconductor structure of claim 1, wherein saidsemiconductor substrate is a semiconductor-on-insulator substrate. 14.The semiconductor structure of claim 1, wherein said U-shaped gatedielectric and said another U-shaped gate dielectric comprise adielectric material having a dielectric constant of greater than 8.0.15. The semiconductor structure of claim 14, wherein said dielectricmaterial having said dielectric constant of greater than 8.0 is selectedfrom the group consisting of HfO₂, ZrO₂, La₂O₃, Al₂O₃, TiO₂, SrTiO₃,LaAlO₃, Y₂O₃, HfO_(x)N_(y), ZrO_(x)N_(y), La₂O_(x)N_(y), Al₂O_(x)N_(y),TiO_(x)N_(y), SrTiO_(x)N_(y), LaAlO_(x)N_(y), Y₂O_(x)N_(y), a silicatethereof, and an alloy thereof, wherein each value of x is independentlyfrom 0.5 to 3 and each value of y is independently from 0 to
 2. 16. Thesemiconductor structure of claim 1, wherein said metal portion and saidanother metal portion include a metal selected from the group consistingof Al, Au, Ag, Cu and W.
 17. The semiconductor structure of claim 1,wherein said first metallic material is selected from Hf, Ti, Zr, Cd,La, Tl, Yb, Al, Ce, Eu, Li, Pb, Tb, Bi, In, Lu, Nb, Sm, V, Zr, Ga, Mg,Gd, Y, TiAl, TaN, a stack thereof, a conductive oxide thereof, aconductive nitride thereof, an alloy thereof, and a combination thereof,and wherein said second metallic material is selected from Pt, Rh, Ir,Ru, Cu, Os, Be, Co, Pd, Te, Cr, Ni, TiN, a stack thereof, a conductiveoxide thereof, a conductive nitride thereof, an alloy thereof, and acombination thereof.